The graft copolymerization of methyl methacrylate onto silk fibers initiated by the ferric chloride-thiourea redox system has been investigated in aqueous medium. The rate of grafting was calculated by varying the concentrations of monomer, initiator, and thiourea, the acidity of the medium, and the temperature. The rate of grafting increases significantly with an increase of [FeCl3] up to 1.25 × 10−3Mand thereafter it slows down. The rate of grafting also increases with an increase of [thiourea] and [HClO4] up to 2.5 × 10−3and 23.325 × 10−1M, respectively, and thereafter it decreases. The graft percezage increases with an increase of the monomer concentration. The effect of some inorganic salts, solvents, and surfactants on the rate of grafting has also been investigated.